This is for 2 windows 2003 box.
Go to administrative tools and then to AD DOMAINS AND TRUSTS. right click on the domain that you have set up and then raise the forest functional to windows server 2003. It will give you a warning about not being able to reverse the setting. Say OK. You lose things that you don't need and gain more. After doing this on both machines go back to AD DOMAIN & TRUSTS, go to properties, and then click on the trust tab. click new trust, then next, then type name of fill in name of domain that you want to connect to then next and then click on two-way trust then click radio button for both this domain and the specified domain, then enter in account information in other domain ( make sure they have permission to create trust. and then click next ... Next on the trust selection complete screen ... nex on the next screen which will talk about success on creating the trust. then confirm the outgoing and no on incoming until the other one is setup up. Then repeat steps on other domain. Once done on both sides of the domain go back to the trust page and then go to properties and validate trust do this for both incoming and outgoing.